In his first track season, at age 16, he finished with 21.20 seconds in the 200 metres and 48.40 seconds in the 400 metres, recording a double win at the Queens County Championships in 1990 and earning MVP honors. After transferring for his final year from Jamaica High to Piedmont Hills High School in San Jose, California, Boldon was selected to the San Jose Mercury News' Santa Clara all-county soccer team. He also continued to sprint, placing third in the 200 m at the CIF California State Meet in 1991. Athletics became his primary focus and he won the Junior Olympic Title that summer in Durham, North Carolina, in 200 m.

Boldon won his first international senior-level medal at the 1995 World Championships, taking home the bronze in the 100 m. At the time he was the youngest athlete ever to win a medal in that event, at 21 years of age. The following year at the 1996 Summer Olympics, he again placed third in the 100 m and 200 m events, both behind world records. In 1997, he won the 200 m at the World Championships in Athens, Greece; his country's first world title in the Athletics World Championships. This made him one of only a few male sprinters to win both a World Junior and World Senior title.

In 1999, Boldon ran 9.86 s twice in the 100 m before sustaining a serious hamstring injury which forced him to miss the World Championships in Seville - the only Championship he missed in his career due to injury.

A silver medal in the 100 m and a bronze in the 200 m were his results of the 2000 Summer Olympics, which was a personal victory, considering his comeback from a career-threatening injury the year before.

In 2001, Boldon tested positive at an early-season relay meet for the stimulant ephedrine, and was given a warning, but was not suspended or sanctioned, since ephedrine is a substance found in many over the counter remedies, and Boldon had been treating a cold. "It is in no way something where the blame is laid on the athlete,” said IAAF General Secretary István Gyulai of the positive result.

Also in 2001, at the World Championships in Edmonton, Alberta, Canada, Boldon finished fourth and out of the medals in the 100 m with 9.98 s, and then ran the second leg of his country's 4 x 100 metre relay, finishing third in the finals. This was Trinidad and Tobago's first 4 x 100 m relay medal in either World or Olympic competition and Boldon states that making national history with this team of young men (the average age of his teammates was 19) was his greatest accomplishment in his career. The colours of his 2001 World Championship medals would change in 2005 as both his placings were improved – he received bronze in the 100 m and the bronze relay medals were upgraded to silver after all the times and performances of the American sprinter Tim Montgomery (who was second in the 100 m and won the 4 x 100 m with the US team) were nullified due to serious doping violations. That brought Boldon's career total to four World Championship medals, to match his four Olympic medals.

Boldon was seriously injured in a head-on crash with a drunk driver in Barataria, Trinidad and Tobago, in July 2002, and never again ran sub-ten seconds in the 100 m or sub-twenty seconds for 200 m; something he had done on 37 separate occasions prior to 2002. In 2006, a judge in Trinidad found that Boldon was not at fault in that accident and he was paid substantial damages as a result. That accident left Boldon with a serious hip injury, and he was a shadow of his former self as a sprinter. In 2004 at the Athens Olympic Games, he failed to advance out of the first round of the 100 m heats but captained his country's 4 x 100 m relay team to their first-ever Olympic final, where they finished seventh.

On 20 April 2008 The Observer published the contents of a letter believed to be by Boldon to John Smith, his former coach, accusing Smith, Maurice Greene of betraying him by obtaining banned drugs without his knowledge, lying about Greene competing without drugs and damaging his own career.[7] But for a quote on the matter to HellenicAthletes.com, a website he wrote for at the time, Boldon has had no further official comment.[8]

At the 1999 World Championships in Seville, Spain, Boldon could not compete due to a serious injury. The British Broadcasting Corporation hired him to do commentary and analysis for their coverage of those Championships. He proved popular with the audience and was invited back as a track-side analyst for the BBC coverage of the U.S. Olympic Track and Field Trials in 2000, from Sacramento, California.

From 2005–2009, Boldon was in the broadcast booth for the U.S. Television network CBS as part of their commentary team for the NCAA Outdoor Track and Field Championships. In June 2007, he made his debut for NBC Sports as an analyst for the 2007 U.S. National Championships, and he also was an integral part of Versus and NBC's coverage of 2007 Osaka World Championships. In 2008, he was the sprint analyst at the US Olympic Track and Field Trials and the 2008 Summer Olympics for NBC Sports.[9] Boldon was widely praised for his NBC work by the press, including the Los Angeles Times, USA Today and The New York Times which called him "one of NBC's best analysts, a blend of athletic smarts, charisma, precise analysis and brashness."[10] In 2010, Boldon joined the only U.S. track and field broadcast team he had not previously been a regular part of, ESPN, after the departure of their long-time analyst, Larry Rawson. In 2012, he continued his role as the NBC track and field analyst for the 2012 Summer Olympics. In 2013, for his 2012 London Olympic commentary, Boldon became the first and only track and field broadcaster in US history to be nominated for a Sports Emmy Award. He was nominated in the category of Outstanding Sports Personality, Sports Event Analyst. Cris Collinsworth, his friend and colleague from NBC Sports' Sunday Night Football, eventually won the Emmy, his fifth win in a row. Alongside Tom Feuer, Boldon has served as a game analyst for Track & Field events for the Pac-12 Network[11]

Boldon was sworn in on 14 February 2006 as a Senator representing the OppositionUnited National Congress following the resignation of former Senator Roy Augustus, who resigned on 13 February in a dispute over the leadership style of then Leader of the Opposition Basdeo Panday. Boldon resigned on 11 April 2007 after 14 months as a senator, also citing issues with Panday's leadership ability.

In 2006, Boldon wrote, produced and directed a 73-minute DVD film entitled Once in a Lifetime: Boldon in Bahrain which documented his voyage with fellow fans and Trinidad and Tobago nationals to the Kingdom of Bahrain, where the country's soccer team, the Soca Warriors, defeated Bahrain 1–0 in a playoff to become the smallest country ever to qualify for the FIFA World Cup, qualifying to play at the Germany 2006 tournament.

Boldon married entertainment executive/manager Cassandra Mills in 1998 after a three-year courtship. Boldon and Mills divorced in 2005. They had no children together. He has two daughters from previous relationships.

In 2000, Boldon was made a sports ambassador by the Republic of Trinidad and Tobago and given a diplomatic passport. He is widely viewed as one of the all-time leading sportsmen in the history of the Caribbean, as well as one of its most internationally recognizable spokesman. When Trinidad and Tobago hosted the 2001 FIFA U-17 World Championship in association football, one of the new stadiums constructed for the tournament was located in Couva and named Ato Boldon Stadium. The only other island sprinter to have a stadium named after him is 1976 Olympic champion Hasely Crawford (Hasley Crawford Stadium located in the capital Port of Spain).

Boldon is a qualified pilot, having earned his private pilot's licence in August 2005. He is a member of AOPA, the Aircraft Owners and Pilot's Association. He resides in Florida.